1. Field of the Invention
This invention relates to an optical recording medium and more particularly relates to a multilayer optical recording medium having laminated a plurality of information-recording layers and a method for recording information in the multilayer optical recording medium.
2. Description of the Related Art
CD and DVD have been widely utilized to date as optical recording media. The recording capacity required by this kind of optical recording medium has been increasing year after year and various suggestions have been made with a view to satisfying this requirement. In the case of such read-only optical recording media as DVD-Video and DVD-ROM, the versions that increase a recording capacity by the impartation of a double-layer structure to the information-recording layer have been reduced to practice.
An optical recording medium, like the DVD illustrated in FIG. 8, has been being reduced to practice. This medium has two substrates which is configured by digging pits on each surface imparting information-recording layers therein. These two substrates are affixed to each other across an interposed spacer layer in such a manner as to enable the information-recording layers to be opposed to each other.
The DVD of hybrid structure having one information-recording layer formatted in the DVD form and the other information-recording layer formatted in the CD form as illustrated in FIG. 9 has been reduced to practice. In this case, for the reason of being adapted to the specifications for DVD and CD, the information-recording layer to be formatted in the DVD form is set at a depth of 0.6 mm from the light incidence surface and furnished with a capacity of 0.7 GB and the information-recording layer to be formatted in the CD form is set at a depth of 1.2 mm from the light incidence surface and furnished with a capacity of 0.7 GB. For the purpose of reading signals recorded in the two information-recording layers, the DVD player necessitates two kinds of optical systems that differ in wavelength and numerical aperture (NA).
Incidentally, with the object of promoting further increase of the recording capacity, new specifications such as of the Blu-ray Disc have been proposed in recent years. In the Blu-ray Disc, information-recording layers are stacked in the depth of about 0.1 mm from the light incidence surface. These information-recording layers are capable of recording 25 GB of information. In the case of the Blu-ray Disc of two-layer structure, for example, one information-recording layer is formed at a position of 0.075 mm from the light incidence surface and the other at a position of 0.1 mm from the light incidence surface.
The optical recording medium of a large capacity is required to have the diameter of the spot of a laser beam used for recording regenerating data decreased to a small size. Thus, it entails the necessity for enlarging the numerical aperture (NA) of the objective lens for focusing the laser beam and shortening the wavelength of the laser beam as well.
The enlargement of the numerical aperture of the objective lens, however, causes a problem that the tilt margin of the laser beam to the optical recording medium, namely the tolerance of the error in the angle of tilt of the optical axis to the optical recording medium, will become very small. Meanwhile, the decrease of the distance from the light incidence surface of the optical recording medium to the information-recording layer induces an increase in the tilt margin. For the sake of preventing the tilt margin from decreasing while enlarging the numerical aperture of the objective lens, therefore, it is effective to decrease the distance from the light incidence surface to the information-recording layer (namely the thickness of the light-transmitting layer). In the Blu-ray Disc, therefore, the thickness of the light-transmitting layer is supposed to be about 100 μm.
When the light-transmitting layer is set at a small thickness and the information-recording layer is consequently formed at a place near the light incidence surface as in the Blu-ray Disc, however, the problem arises that finger marks, rubbish, etc. are liable to exert a bad influence on the operation of recording and regenerating information. When the information-recording layer is further added to the specification of the Blu-ray Disc, for example, since the information-recording layer must be placed at a distance within 0.1 mm, particularly within 0.75 mm, from the light incidence surface, the problem arises that the finger marks adhering to the surface of the medium exert a still larger influence tending toward deteriorating the signal-regenerating property.
Meanwhile, an effort to increase the distance of the information-recording layer from the light incidence surface as much as possible conversely entails the problem that the individual information-recording layers inevitably suffer from interference of signal because the increased distance results in concentrating a plurality of information-recording layers into the neighborhood of 0.1 mm from the light incidence surface and decreasing the distance between the adjacent information-recording layers.